The present invention relates to a method for manufacturing a semiconductor substrate, and more particularly to a method for manufacturing a group III nitride semiconductor substrate, which is used in a semiconductor laser device for emitting light of a short wavelength such as blue or violet, or in a transistor capable of operating at a high temperature.
A group III nitride semiconductor that is expressed by the general formula BxAlyGazIn1-x-y-zN (0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦x+y+z≦1) (hereinafter referred to simply as “group III nitride semiconductor”) is a material used in optical devices for wavelengths ranging from red to ultraviolet, and is expected to be used in light-emitting devices and light-receiving devices. Unlike other group III-V compound semiconductors such as GaAs or InP, a group III nitride semiconductor such as GaN is difficult to obtain in the form of a high-quality, large-area, independent substrate. Therefore, a group III nitride semiconductor substrate of a satisfactory quality has been produced by depositing a group III nitride semiconductor layer on a heterogeneous substrate having a different lattice constant such as sapphire or SiC.
However, if a group III nitride semiconductor layer formed on a sapphire substrate is used in a semiconductor laser device or a transistor, it is necessary to form all electrodes on the group III nitride semiconductor layer because the sapphire substrate is an electrically non-conductive, or insulative, substrate. This complicates the manufacturing process and lowers the yield of a device that is made of a group III nitride semiconductor.
When an SiC substrate is used, even though an SiC substrate is conductive, a potential barrier is likely to be formed at the interface between the SiC substrate and the group III nitride semiconductor layer. Thus, when an electrode is formed on the lower surface of the SiC substrate, the operating voltage will be high.
In view of the above, attempts have been made in the art to obtain an independent group III nitride semiconductor substrate by forming a group III nitride semiconductor layer on a heterogeneous substrate such as a sapphire substrate or an SiC substrate and then separating the group III nitride semiconductor layer from the heterogeneous substrate.
A conventional method for manufacturing a group III nitride semiconductor substrate will now be described with reference to FIG. 2A to FIG. 2C.
First, in the step of FIG. 2A, a sapphire substrate 1 having a diameter of 2 inches and a thickness of 400 μm and whose upper surface is oriented in the (0001) direction is provided. Then, the sapphire substrate 1 is carried into a metal-organic chemical vapor deposition (hereinafter referred to as “MOCVD”) reactor, in which it is heated to a temperature of about 1100° C. in a hydrogen atmosphere and held at that temperature for 10 minutes so as to clean the surface of the sapphire substrate 1. Then, the substrate temperature is decreased to about 550° C., and ammonium and trimethylgallium are introduced into the reactor so as to form a GaN buffer layer (not shown) having a thickness of about 200 Å on the sapphire substrate 1. Then, the supply of trimethylgallium is once stopped, and the substrate temperature is increased to about 1050° C. in the hydrogen/ammonium mixed atmosphere, after which the trimethylgallium supply is resumed so as to form a GaN layer 2 having a thickness of 10 μm on the sapphire substrate 1.
Then, in the step of FIG. 2B, the obtained substrate is taken out of the reactor, and a pulse laser beam of a YAG tertiary harmonic wave (wavelength: 355 nm) is illuminated from the lower surface of the sapphire substrate 1 onto the lower surface of the GaN layer 2 via the sapphire substrate 1 so that the entirety of the lower surface of the GaN layer 2 is scanned, with the substrate being heated to about 600° C. on a heating stage. Note that an arrow in FIG. 2B represents the laser beam. While the sapphire substrate 1 transmits the laser beam having a wavelength of 355 nm therethrough, the GaN layer 2 strongly absorbs the laser beam. Thus, the portion of the GaN layer 2 that is irradiated with the laser beam is heated by absorbing light so as to be decomposed into a metal (Ga) and a nitrogen gas. Eventually, the lower portion of the GaN layer 2 is decomposed.
Then, in the step of FIG. 2C, the sapphire substrate 1 and the GaN layer 2 are separated from each other so as to obtain an independent GaN substrate 4a. 
However, with the conventional method described above, the GaN substrate 4a separated from the sapphire substrate 1 is substantially warped so as to protrude away from the sapphire substrate 1. When an SiC substrate is used, instead of the sapphire substrate 1, the GaN substrate 4a is substantially warped so as to protrude toward the SiC substrate.
In any case, where an independent group III nitride semiconductor substrate is obtained by forming a group III nitride semiconductor film on a heterogeneous substrate such as a sapphire substrate or an SiC substrate and then separating the group III nitride semiconductor film from the heterogeneous substrate, the group III nitride semiconductor substrate is deformed.
Deformation of the group III nitride semiconductor substrate makes it difficult to handle or process the independent group III nitride semiconductor substrate. Moreover, in a case where the independent group III nitride semiconductor substrate is used as a substrate on which another group III nitride semiconductor layer is deposited, if a group III nitride semiconductor substrate that is deformed as described above is used, it will be difficult to uniformly heat the entirety of the group III nitride semiconductor substrate in the reactor, and to uniformly grow a crystal.
Moreover, since a group III nitride semiconductor substrate inherently has a force of deforming itself as described above, a group III nitride semiconductor substrate is likely to be cracked during the step of illuminating a pulse laser beam, and it is difficult to obtain a large-area, independent group III nitride semiconductor substrate.